An actuator incorporated into a robot arm or the like is known in which a wave gear device is provided as a reduction gear mechanism for reducing a rotational speed of a motor, which is the drive source of the actuator. The wave gear device is substantially composed of a rigid internally toothed gear, a flexible externally toothed gear disposed inside the rigid internally toothed gear, and a wave generator fitted inside the flexible externally toothed gear. A typical wave generator has an elliptical profile, which elliptically flexes the flexible externally toothed gear and partially mesh it with the rigid internally toothed gear. A high-speed rotating input shaft of a motor or the like is connected to the wave generator. The meshing locations of the two gears move circumferentially when the wave generator rotates at high speed, and the two gears rotate relative to each other in correspondence with the difference in the number of teeth between the two gears. Generally, the rigid internally toothed gear is fixed in place, and reduced-speed rotational output is obtained from the flexible externally toothed gear.
In a wave gear device, ratcheting (gear jumping) occurs between the two gears when torque overload acts during speed reduction, speed increase, or stoppage. When ratcheting has occurred, the ratcheting torque of the two gears is considerably reduced, plastic deformation occurs in each part, vibrations are increased or abnormal abrasion is generated, and other adverse effects are observed. It is difficult to reuse the wave gear device because of such adverse effects; even if the phase is adjusted after ratcheting has occurred.
A wave gear device is proposed in Patent Document 1 in which a safety function is provided for the case in which an excessive load is applied. In the wave gear device of the disclosure, a constriction force is applied to the housing to fit an internally toothed gear via an elastic member, and slippage is allowed to occur in the fitted location when an excessive load is applied so that the excessive load is not applied to the gear.    [Patent Document 1] JP-A 2005-54981